Mass spectrometry is used quite commonly to characterize polymers. However, it requires an ionization technique that causes little or no fragmentation of the sample molecules during ionization and a means of detecting the ions that yields high resolution in the high-mass range.
Field desorption, fast-atom bombardment and laser desorption are the ionization techniques most commonly used but all have difficulties with various polymers and field desorption and fast-atom bombardment cannot be used with insoluble materials. Laser desorption is thus the preferred technique especially for insoluble polymers.
The detection of ions can be accomplished with quadrupole, sector, time-of-flight, and Fourier transform mass spectrometers. However, quadrupole instruments are limited to detection of ions of low mass and, thus, are of little use for detecting polymers and sector instruments cannot be used readily with the pulsed ionization techniques used to ionize polymers. The time-of-flight instruments work well for detecting high-mass ions produced by pulsed ionization techniques but have inherent limitations regarding resolution. As a result, it is preferred to use laser desorption/Fourier transform mass spectrometry (LD/FTMS) in the field of a superconducting magnet.
The commonly assumed mechanism of laser desorption of thin films by a pulsed CO.sub.2 laser involves the heating of the sample to form an ionized plasma. If heating of the sample by the laser pulse and the subsequent cooling by expansion into a vacuum chamber is sufficiently rapid, then molecules are moved into the gas phase with little or no bond breakage. Ionization may occur either as the sample molecules are removed from the polymer surface on a probe tip or by the interaction of the molecules with other particles in the plasma. The addition of a cationic species, such as Na.sup.+ or K.sup.+, to the polymer sample on the probe tip is known to enhance the formation of the sodium or potassium adduct molecular ion. Thus, the severity of the desorption conditions needed to produce ions, and therefore the extent of fragmentation, can be decreased by codepositing KBr onto the probe tip with polymer solutions. This procedure does not work with all polymers. Therefore, other techniques are needed for ionization of polymers which fragment readily and of the so-called "intractable" polymers which are insoluble, resistant to oxidation, thermal degradation and radiation.